Various processes are known for the conversion of gaseous hydrocarbonaceous feedstocks, especially methane from natural sources, for example natural gas, associated gas and/or coal bed methane, into liquid products, especially methanol and liquid hydrocarbons, particularly paraffinic hydrocarbons. At ambient temperature and pressure these hydrocarbons may be gaseous, liquid and (often) solid. Such processes are often required to be carried out in remote and/or offshore locations, where no direct use of the gas is possible. Transportation of gas, for example through a pipeline or in the form of liquefied natural gas, requires extremely high capital expenditure or is simply not practical. This holds true even more in the case of relatively small gas production rates and/or fields. Re-injection of gas will add to the costs of oil production, and may, in the case of associated gas, result in undesired effects on crude oil production. Burning of associated gas has become an undesirable option in view of depletion of hydrocarbon sources and air pollution. A process often used for the conversion of carbonaceous feedstocks into liquid and/or solid hydrocarbons is the well-known Fischer-Tropsch process.
The Fischer-Tropsch process can be used for the conversion of hydrocarbonaceous feed stocks into liquid and/or solid hydrocarbons. The feed stock (e.g. natural gas, associated gas and/or coal-bed methane, residual (crude) oil fractions or coal) is converted in a gasifier, optionally in combination with a reforming unit, into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas).
The synthesis gas is then fed into a Fischer-Tropsch reactor where it is converted in one or more steps over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight modules comprising up to 200 carbon atoms, or, under particular circumstances, even more.
The hydrocarbons formed in the Fischer-Tropsch reactor proceed to a hydrogenation unit, preferably a hydroisomerisation/hydrocracking unit, and thereafter to a distillation unit.
The feed stock, being a natural product, includes a number of impurities. These impurities are not removed by the partial oxidation of the feed stock to form the syngas. The main impurities are hydrogen sulphide, carbon dioxide, mercaptans, and other sulphur containing compounds such as thiophenols and aromatic sulphur compounds, as well as carbonyl sulphide, sometimes also termed carbon oxysulphide, and generally known by the term COS. It is generally desired to remove some if not all of these impurities prior to use of the syngas in a Fischer-Tropsch reaction vessel, as at least the sulphur compounds reduce the effectiveness of the generally used catalysts in Fischer-Tropsch processes, by poisoning the catalysts.
The removal of sulphur-containing compounds from gas streams comprising such compounds has always been of considerable importance in the past and is even more so today in view of continuously tightening environmental regulations. This holds for combustion gases as obtained in the combustion of organic compounds as coal, as well as for natural gas streams to be used for e.g. the preparation of synthesis gas and for residential use or to be transported as liquid natural gas.
Sulphur contaminants in natural gas streams include, beside hydrogen sulphide, carbonyl oxysulphide, carbonyl sulphide and mercaptans. Mercaptans, due to their odorous nature, can be detected at parts per million concentration levels. Thus, it is desirable for users of natural gas to have concentrations of mercaptans lowered to e.g. less than 5, or even less than 2 ppmv, and total concentration of sulphur compounds to e.g. less than 30 or, preferably, less than 20 ppmv, e.g. 15 or 10 ppmv. Sales gas specifications often mention total sulphur concentrations less than 4 ppmv.
Numerous natural gas wells produce what is called “sour gas”, i.e. natural gas containing hydrogen sulfide, often in combination with mercaptans, the total amount of sulphur compounds being present in concentrations that makes the natural gas unsuitable for direct use. Considerable effort has been spent to find effective and cost-efficient means to remove these undesired compounds.
A number of processes are known for the removal of sulphur compounds from gas streams such as natural gas. These processes are based on physical and/or chemical absorption, solid bed adsorption and/or chemical reaction. Physical and/or chemical absorption processes suffer from the fact that they frequently encounter difficulties in reaching the low concentration of the undesired sulphur compounds, unless (extremely) large reactors are used. Solid bed adsorption processes suffer from the fact that they are only able to adsorb limited amounts of undesired compounds, while regeneration is relatively cumbersome. Especially large solid beds take relatively large amounts of time for regeneration and disproportionately large amounts of regeneration gas are needed. In addition, the solid adsorption beds usually also remove water, thus requiring frequent regenerations.
Sulphur compounds, especially hydrogen sulfide in combination with mercaptans, and optionally carbonyl sulphide, may be removed from gas streams, especially natural gas streams, by a combined process known from WO2004/047955, in which in a first physical/chemical absorption step, most of the hydrogen sulphide, and a part of the mercaptans is removed, and in a second solid adsorption step, the remaining hydrogen sulphide and the remaining mercaptans and other sulphur compounds are removed.
The above process uses the well-proven physical/chemical absorption process. Such a process has been described in for instance GB 2,103,645 and GB 2,103,646, incorporated herein by reference. Almost all hydrogen sulphide is removed in a very efficient way. As only part of the mercaptans has to be removed in the first step, the process avoids the use of very large and inefficient reactors. In the second step a relatively small solid adsorption bed can be used to remove the remaining part of the mercaptans. This is due to the fact that almost all hydrogen sulphide has already been removed in the first step together with part of the mercaptans. Regeneration of such a bed is not very laborious or cumbersome. Thus, the above combination of sulphur removal processes results in an overall efficient removal of hydrogen sulphide, mercaptans and optionally part of the carbon dioxide and carbonyl sulphide, while avoiding the disadvantages of only one technology or other technologies. In addition, treating the regeneration gas of the solid bed adsorber in a dedicated absorber optimises the process. The loaden solvent of the dedicated absorber is then regenerated in the same regenerator as is used for the main process.
Meanwhile, the natural gas may also contain varying amounts of carbon dioxide. Presently, the carbon dioxide is generally removed, and may be simply vented to atmosphere. However, there is increasing legislation about the emission levels of carbon dioxide.